It is common for hydraulic pipes and other systems or systems equipment to be routed through the inside of an aircraft wing and to be attached to its inner surface. Similarly, components such as heat exchangers often need to be mounted to the inside surface of the wing. As the wing is effectively a fuel tank, the pipes, systems or other components may be immersed in fuel or, at the very least, exposed to fuel vapour. The fuel can then be used to cool the hydraulic system which can operate at at up to 120 deg. C in normal operation. Therefore, it is important for the fuel tank to be electrically and thermally insulated to stop or reduce the amount of heat being conducted into the wing structure which, if made from carbon composite, can start to degrade at around 70 deg. C.
It is conventional for pipes, systems and components to be mounted on brackets that are mounted on the inside of the skin of an aircraft, in particular on the inner surface of the skin covering the wing using nuts and bolts that pass through holes in the skin and the component or bracket. Ideally, these bolts are a clearance fit through the holes in the skin and the holes in the component or bracket to ensure that the components or pipes can be easily released by undoing the bolts during maintenance or replacement.
When lightning strikes an aircraft wing, the metal outer skin acts as a Faraday cage and so protects the components mounted within the wing from electromagnetic damage and, more importantly, dissipates the energy from the lightning strike away from the fuel. However, it has recently become more common to manufacture the skin of an aircraft wing out of a carbon composite material rather than metal. This reduces its ability to dissipate the energy generated as a result of a lightning strike.
The risk due to lightning is due to either a direct strike or current transfer. All fasteners that may be directly struck by lightning or may form part of a current transfer path which are inside fuel tanks or penetrate the fuel tank boundary require protection against ‘out-gassing’ (also known as ‘pressure sparking’). Current transfer from the bolt shank of a fastener to the surrounding hole of the composite wing through which it passes causes either arcing or material degradation giving rise to a jet of sparks (plasma). Protection is also required against voltage sparking in which a spark jumps a gap between conductive parts and, thermal sparking in which current density causes local heating and out gassing due to defective protective inter-fastener layer (e.g. under the nut of a fastener).
To mitigate against the above issues, mounting brackets are permanently attached to the inner surface of the outer skin of the wing of an aircraft using bolting and installation techniques which protects the skin and bolt hole and guards against the hazards referred to above. One fastener assembly is known from the Applicant's own earlier patent application No. 1101311.7.
However, there is still a risk that lightning will continue to follow a path to the system (such as a hydraulic pipe) mounted to the bracket. Therefore, adequate electrical insulation is also required to prevent the lightning current flow along the system that could result in a spark.
An electrostatic charge can be accumulated by fluid flowing in pipes/across components. Therefore, systems within the fuel tank are bonded in a particular way so that the accumulated charge is conducted out of the fuel tank. In the case of the hydraulic installation, this is through the use of highly resistive fittings at the tank boundaries, as these provide sufficient electrical conductivity to prevent the build up of electrostatic charge, but interrupt lightning current flow. Therefore, where the hydraulic installation is attached or passes through structure it must either be in a manner that is either completely isolated or highly resistive. It is important for the hydraulic installation to follow this philosophy throughout to prevent any sparking.
The present invention therefore seeks to provides a fastener that mounts a component to a bracket attached to an inner surface of a skin of an aircraft wing which is isolated or highly resistive to ensure that current flow due to lightning is suppressed.
Furthermore, some system installation equipment thermally expands as part of its normal operation. However, the wing structure to which it is mounted does not necessarily expand at the same rate. So, in addition to the requirement for the equipment to be thermally and electrically isolated from the structure, provision also needs to be made to allow the attachment to slide or deflect to accommodate thermal expansion effects whilst maintaining thermal and electrical insulation requirements.